The present invention relates to an improved device for achieving an excellent measurement of the flow-rate of a gas in a tube, without the need for any correction algorithm, by evaluation of the transit time of an ultrasound system by virtue, for example, of the use of reflectors with parabolic structure and of arrangements which eliminate all of the disturbances and errors which render measurements problematical in known apparatus.
Three basic types of disturbance in the achievement of a correct signal during the measurement of the flow-rate of a gas are known:
a) the distribution of the velocity of the gas in the measurement cross-section, which may be laminar with a profile varying from elliptical to flat according to the distance from the inlet and the Reynolds number, or turbulent with a greater or lesser degree of disturbance due also to asymmetry; a system of ultrasound beams passing through portions of gas having different velocities in fact reaches the receiver with delays in time, normally measured in terms of phase, which are varied and erratic to the extent of not only causing measurement errors but even of cancelling out the signal when the various contributions arrive with mean values in phase opposition;
b) diverging and not flat propagation of the ultrasound emission which causes reflections on the walls of the measurement tube with the result of obtaining various sound paths between the shortest, which corresponds to the axial direction of the tube to be measured, and the longest which involve one or more reflections on the walls; in this case also the errors and interference may cancel out the measurement signal;
c) non-uniform distribution of the energy within a diverging or parallel ultrasound beam; this leads to an error in the integration of the signal received in relation to the measurement of the transit time when portions of the ultrasound beam having unequal energy levels pass through the gas flow which has regions characterized by different velocities; the integration of the signals thus produces a weighted mean velocity in which the regions through which ultrasound beams with greater energy levels pass have greater weight.
Clearly, the three types of disturbance in combination make the problem of achieving accurate measurements very difficult, particularly when there are large variations in the flow-rate and also when the speed of the sound is subject to variations connected with the type and temperature of the gas being measured.
In order to reduce the effects of the first two types of disturbance, the practical solutions available in the prior art generally adopt geometry of the gas-flow tube based on the flat or annular slit principle, in which the spacing of the walls is such as to reduce the angles of reflection to an extent such as to render the paths of the reflected sound little different from the direct paths without any reflection, as well as achieving a reduction in the Reynolds number by virtue of the slot so as always to have a laminar condition, in order to limit problems connected with the distribution of the velocities in the measurement tube.
Many of these solutions involve surfaces having indentations provided for absorbing the reflected ultrasound beams which may interfere with the correctness of the measurement and with the signal/noise ratio which, however, from the energy point of view, becomes gradually worse as some of the sound energy is absorbed by the surfaces.
In order to reduce the angle of spread of the ultrasound beam transmitted and thus to reduce the energy which is lost and also harmful in terms of interference, in the known solutions, the transmitters/receivers generally used are characterized by diameters several times larger than the wavelength of the sound in the gas, and thus have quite high natural frequencies, for example, as an order of magnitude, of about 200 kHz.
As a consequence of the less than optimal solution of the problems relating both to the unevenness of the velocity distribution and to the fact that the propagation of the sound emission is not flat with parallel paths, as well as to the non-uniform distribution of the energy within the ultrasound beam performing the measurement, the known apparatus generally has measurement errors, as the flow-rate varies, which necessitate experimentally-based correction calculations, whilst the unfavourable signal-noise ratio leads to power consumption which is not optimized.
In French patent application FR A-2683046, the reflections are arranged so as to prevent the second type of disturbance mentioned above with the use of a geometry based on an ellipsoid of rotation in which the paths with a single reflection between the two focus points of the ellipse are uniform.
However, although on the one hand, this solution solves the problems connected with the second type of disturbance, on the other hand, it does not solve the problems connected with the first and third types of disturbance mentioned above which are more than sufficient to render the measurements uncertain. Moreover, in order to eliminate direct transmission without reflections, the device of the French document FR A-2683046 involves the insertion of a central solid which is also necessary in order to achieve a uniform gas-flow cross-section in which the measurements take place, and this gives rise to a considerable loss of energy in addition to the spurious reflections which nevertheless arise and interfere with the measurement.
The main object of the present invention is to overcome the disadvantages of the known devices described above, achieving a high degree of intrinsic accuracy without the need for software corrections, combined with a high signal/noise ratio, such that it is possible to reduce power consumption and the size of the batteries required for the necessary autonomy.
Another object of the present invention is to overcome the problems of the known solutions connected with the elimination of variable and irregular profile distribution by propagating the ultrasound beam not only parallel to the generatrix of the measurement tube and with a flat front, but also with a uniform energy distribution throughout the cross-section, so as to achieve a perfect integration of all of the components of the movement of the gas, in the axial direction alone. Only in these conditions does the measurement in fact become independent of the conditions of movement of the gas and also insensitive to any asymmetry of the flow created by ducting upstream of the inlet.
A further object of the present invention is to provide a gas-flow measuring device which can be produced easily and quickly, and which comprises a limited number of elements so as to reduce its cost at the manufacturing stage.
In order to achieve the objects indicated above, the subject of the present invention is an improved system for measuring the flow-rate of a gas by means of ultrasound, comprising a tubular element in which a plurality of openings are formed to allow one or more gas-flows to pass inside the tubular element, and two or more means for transmitting/receiving an ultrasound beam, characterized in that it further comprises two or more means for focusing ultrasound beams, the transmitting/receiving means transmitting and receiving, respectively, the ultrasound beams which pass through or are reflected by the focusing means and pass through the one or more gas-flows, inside the tubular element.
One embodiment of the present invention comprises the sound equivalent of two optical lenses, of which the first transmission lens transforms a diverging beam originating from a first transmitting/receiving means into a parallel beam which passes through the entire gas-flow measurement tube, whilst another downstream lens with a symmetrical arrangement focuses the beam at a point where a second transmitting/receiving means is located. With these characteristics, it is possible to achieve the maximum ratio between signal and noise.
One of the main advantages of the present invention consists of the production of an ultrasound beam with an energy distribution which, by virtue of the focusing means, is very uniform throughout the cross-section, in contrast with the arrangements of the prior art which mostly provide for transmission with a beam which diverges as little as possible. This advantage can be achieved, for example, with the use of transmitting/receiving means characterized by a spherical, non-focused emission, substantially by virtue of a diameter to wavelength ratio, for example, of less than about 1 and of relatively low resonance frequencies, typically, but not exclusively of about 40 kHz.
In a further embodiment of the present invention, the means for focusing the sound-wave beam comprise, for example, instead of materials with different acoustic refraction indices which are difficult to produce, two portions of parabolic mirrors, with two reflections which give rise to an intermediate ultrasound beam with parallel generatrices. It is thus possible to achieve a functionality in the acoustic field which is completely identical to the optical functionality with the advantage, in addition to simplicity, of permitting acoustic alignment by optical means. Naturally, focusing means with shapes other than those indicated above may also be used, as long as the objects of the present invention can be achieved.
A further advantage of the present invention consists of the use of an optical check for verifying the existence, in the measurement tube, of a system of optical rays parallel to the axis of the tube, which thus confirms that the propagation of the ultrasound beam is actually flat and thus suitable for correctly integrating all of the axial contributions to the velocity of the fluid along the measurement tube. The uniformity of the energy throughout the measurement cross-section can also be checked easily, for example, but in a non-limiting manner, by optical means.
Another advantage of the present invention consists of the use of two reflections on the above-mentioned parabolic surfaces which permit identical paths for the entire beam transmitted, which is better than in the arrangement which provides for the use of the lenses and is free of the problems of a continuously variable cross-section in arrangements based on ellipsoidal geometry with a single reflection.